Planarization process for the manufacturing of integrated circuits, particularly for non-volatile semiconductor memory devices

Abstract: A planarization process for the manufacturing of integrated circuits, particularly for non-volatile semiconductor memory devices, comprises the steps of: forming a first layer (12) of undoped oxide acting as a barrier layer over a semiconductor substrate (3) wherein integrated devices (M,MC1,MC2) have been previously obtained; forming a second layer (13) of oxide containing phosphor over the first undoped oxide; forming a third layer (14) of oxide containing phosphor and boron over the second oxide layer, the concentration of phosphor being lower than or equal to the concentration of boron; performing a thermal process at a temperature sufficient to melt the third oxide layer (14), to obtain a planar surface.

Description: The present invention relates to a planarization process for the manufacturing of integrated circuits, particularly for non-volatile semiconductor memory devices. Dielectric thin films are used in integrated circuit, for example to provide insulating layers between different electrically-conductive layers: in MOS technology, for example, they are used to electrically insulate the polisilicon gates of the MOS transistors from the overlaying metal interconnection lines. Dielectric thin films are commonly represented by oxides layers doped with boron and phosphorus (known as "Boron-Phosphorus Silicate Glass" or "BPSG") which are deposited by means of the Chemical Vapour Deposition ("CVD") technique. BPSG films allow to achieve a plane surface over the substrate wherein the devices are integrated. To this purpose, after their deposition the oxide films are submitted to a thermal process ("reflow"), in furnaces or in Rapid Thermal Annealers ("RTA"), at temperatures near their melting point (800-1000 °C), to obtain a planar surface. The melting-point temperature of such films depends on their doping level and, for a given total impurity concentration (sum of the phosphorus and boron concentrations), decreases for higher boron concentrations. Some integrated circuits, such as non-volatile memory devices (EPROMs and Flash EPROMs), require BPSG films with high phosphorus concentration (typically over 6%), since phosphorus atoms act as gettering centers for high-mobility metal impurity species which are responsible for the performance degradation of the memory cells (leakage of charges stored in the floating gates, causing the loss of the stored information); a planarization process such this is difficult for non-volatile semiconductor memory devices such as EPROM and Flash EEPROM. Since to achieve stable BPSG films the total impurity concentration shall not exceed a given value, the boron concentration must be low (less than 3%), and the resulting melting-point temperature is too high and not compatible with the new-generation integrated devices: this prevents an effective planarization of the surface of the BPSG film from being achieved. EP-A-0280276 discloses an interlayer insulation film between memory elements in an UV erasable nonvolatile semiconductor memory device and an upper metal wiring layer, the interlayer insulation film comprising a thermal oxide film, formed on a semiconductor substrate, a phosphorus doped oxide (PSG) film, formed on the thermal oxide film, and a boron and phosphorus doped (BPSG) film, formed on the PSG film. The PSG film improves the UV transmissive properties of the insulation film; after the deposition of the BPSG film, it is made to reflow by means of a thermal process at 900 °C. Such an interlayer insulation film has two major drawbacks: first, since the PSG film has a melting temperature substantially higher than that of the BPSG film, it does not reflow at 900 °C, so it does not contribute to achieve a higly planar interlayer insulation film. Second, if cleaning of the bottom of the contact windows is carried out by wet etching, by means of hydrofluoric acid-based solutions, the contact windows are deformed, since the PSG film etch rate is substantially higher than that of the BPSG film. EP-A-376479 discloses also an interlayer insulation film between conductors, comprising a thermal oxide film, and two highly planar BPSG films. In view of the state of the prior art described, the object of the present invention is to provide a planarization process suitable to obtain dielectric thin films with highly planar surfaces without the need of high-temperature thermal steps, and which is therefore suitable to be used in the fabrication of particular integrated circuits, such as non-volatile semiconductor memory devices, which do not allow high-temperature thermal steps. According to the present invention, such object is attained by means of a planarization process for the manufacturing of highly-planar interlayer dielectric thin films in integrated circuits, particularly in non-volatile semiconductor memory devices as set forth in claim 1. The features of the present invention will be made more evident by the following detailed description of a particular embodiment, illustrated as a non-limiting example in the annexed drawings, wherein: Figures 1 to 5 are cross-sectional views taken at intermediate steps of a planarization process according to the present invention applied to the fabrication of a MOSFET; Figures 6 to 9 are cross-sectional views showing further successive steps of the fabrication process of the MOSFET; Figures 1A to 5A are cross sectional views taken at the same intermediate steps of Figures 1-5, showing the planarization process according to the invention applied to the fabrication of non-volatile memory cells. The planarization process according to the present invention will be described taking as an example the fabrication of a MOS integrated circuit, in particular a non-volatile semiconductor memory device. Figures 1 and 1A respectively illustrate a MOSFET M and two stacked-gate non-volatile memory cells MC1, MC2, and can be considered to represent two distinct regions, i.e. a peripheral region and a memory matrix region, of a non-volatile semiconductor memory device. Both the MOSFET and the two memory cells have been obtained in a conventional way, exploiting any known manufacturing process. The MOSFET M comprises in a per-se known way a source region 1 and a drain region 2 obtained by introducing dopant species into a semiconductor substrate 3; a polysilicon gate 4, separated from the substrate 3 by a thin gate oxide layer 5, extends between the source and drain regions 1 and 2; thick field oxide areas 6 are provided around the MOSFET M. The two memory cells MC1 and MC2 comprise a common source region 7, and drain regions 8 obtained in the semiconductor substrate 3; a polysilicon floating gate 9, separated from the substrate 3 by the thin gate oxide layer 5, extends between the drain region 8 and the source region 7 of each memory cell MC1 and MC2; an interpoly dielectric layer 10 is superimposed on and autoaligned with the floating gate 9, and a polysilicon control gate 11, representing a so-called "word line" of the memory matrix, is further superimposed over the interpoly dielectric layer 10. The polysilicon gate 4 of the MOSFET M and the polysilicon control gates 11 of the memory cells MC1 and MC2 constitute first-level electrical interconnection lines. According to the planarization process of the present invention, a first undoped oxide layer 12 is deposited over the whole integrated circuit surface (Figs. 2 and 2A), for example by means of the known Low-Pressure Chemical Vapor Deposition ("LPCVD") technique, at atmospheric pressure (Atmospheric-Pressure CVD or "APCVD") or sub-atmospheric pressure (Sub-Atmospheric-pressure CVD or "SACVD"); Plasma-Enhanced CVD ("PECVD") can also be used. The thickness of the first undoped oxide layer 12 is optimized to assure suitable barrier properties against the diffusion of dopant species from the overlaying layers into the semiconductor substrate 3, which could alter the desired doping levels of the previously obtained source and drain regions 1, 2, 7 and 8: a good compromise is generally achieved by making the first oxide layer 12 about 100 nm thick or lower. The thickness of such oxide layer can however vary from 50 nm to 300 nm. A second oxide layer 13 is then deposited over the first undoped oxide layer 12 by means of the already mentioned APCVD or SACVD techniques, introducing in the reaction chamber both gaseous species, such as SiH 4 , PH 3 , B 2 H 6 , and liquid species, such as Si(OC 2 H 5 ) 4 (Tetra-Ethyl-Ortho-Silicate or "TEOS"), B(OCH 3 ) 3 (Tri-Methyl-Borate or "TMB"),P(OCH 3 ) 3 (Tri-Methyl-Phosphite or "TMP"). This second oxide layer 13 is doped with boron and phosphorus, but the concentration of phosphorus is higher than that of boron; the concentration of phosphorus is chosen in such a way as to achieve a good gettering action for the high-mobility metal ions; given the phosphorus concentration, the concentration of boron is chosen to achieve a stable deposited film: for a concentration of phosphorus higher than 6%, the boron shall be present in a concentration lower than 3%. The thickness of the second oxide layer 13 is determined on the basis of the amount of phosphorus that is necessary to assure the desired gettering action, and shall be as thin as possible; typical values range from 80 nm to 500 nm. If the thickness of the second oxide layer 13 approaches to high values (> 300 nm), then a preliminary thermal step can be carried out in order to improve the surface topography. Since however the second oxide layer 13 contains more phosphorus than boron, its resulting melting point temperature can be higher than the maximum temperature allowed by many manufacturing processes (such as the manufacturing process for non-volatile memory device under consideration), and then hardly to be implemented. A third oxide layer 14 is then deposited, by means of the already cited CVD techniques, over the second oxide layer 13 (Figs. 4 and 4A); the third oxide layer 14 is also doped with boron and phosphorus, but the concentration of boron in this layer is equal to or higher than the concentration of phosphorus; the concentration of boron is chosen in such a way as to lower the melting-point temperature of the oxide layer 14 to the desired value: if for example the maximum temperature allowed by the particular process is 900 °C, a concentration of boron of about 4-5% is suitable; given the boron concentration, the concentration of phosphorus is chosen to achieve a stable film (in the example 4%). The thickness of this third oxide layer 14 depends on the thickness desired for the overall triple-layer composed by the first, the second and the third oxide layers 12, 13 and 14 (generally ranging from 500 nm to 1500 nm), and on the thickness of the first and second oxide layers 12 and 13. A thermal process is then performed, to cause the reflow of the third oxide layer 14, so that the surface of the integrated circuit can be planarized; due to the high concentration of boron, the third oxide layer 14 has a low melting-point temperature, so that temperatures lower than 900 °C, compatible with the manufacturing process of non-volatile memory devices, are sufficient for the reflow to occur. After this thermal process, the degree of planarization of the surface of the integrated circuit is quite high (Figs. 5 and 5A). The triple oxide layer 12, 13 and 14 is then selectively removed down to the surface of the substrate 3 to open contact windows 15 (Fig. 6). After this step, dopant species are implanted through the contact windows 15 and diffused into selected regions of the substrate 3 to assure the formation of good ohmic contacts; during the thermal diffusion step, a thermal oxide layer 16 is unavoidably formed over the exposed regions of the substrate 3 (Fig. 7). To remove the thermal oxide layer 16, a cleaning step must be perfomed: a conventional technique provides for using wet-etching solutions of diluted hydrofluoridic acid (HF) (Fig. 8). The use of a double BPSG layer (i.e. the second and third oxide layers 13 and 14) over a barrier oxide layer 12 is advantageous in that the second and the third layers of oxide 13 and 14 are etched at almost the same rate by the wet-etching solutions. On the other hand if the second oxide layer 13 contained only phosphorus then it would be overetched with respect to the third oxide layer 14, because of the large difference between the two etch rates. In this way the contact window profile 15 would result fully deformed and then unfit to be filled by the following metal layer. If however the thermal oxide layer 16 is removed by dry-etching, instead of wet-etching, the second oxide layer 13 can contain only phosphorus, and not boron (in this case the second oxide layer 13 will be a Phospho-Silicate Glass or "PSG"). A metal interconnection layer 17 is then deposited over the planarized surface of the integrated circuit, and is selectively removed to give rise to the desired pattern of metal interconnection lines. Even if the planarization process according to the present invention has been described in connection with the manufacturing of a MOS integrated circuit, particularly a non-volatile memory device, it can be used for the fabrication of integrated circuits in general. The planarization process according to the invention is particularly suitable to obtain highly planar dielectric films which do not require thermal process at high temperatures.

Claims: A planarization process for the manufacturing of highly-planar interlayer dielectric thin films in integrated circuits, comprising the steps of: a) forming a first layer (12) of undoped oxide acting as a barrier layer over a semiconductor substrate (3) wherein integrated devices (M,MC1,MC2) have been previously obtained; b) forming a second layer (13) of oxide containing phosphorus and boron over said first undoped oxide layer (12) and acting as a gettering layer, c) forming a third layer (14) of oxide containing phosphorus and boron over said second oxide layer (13), d) performing a thermal process at a temperature sufficient to melt the third oxide layer (14), to obtain a planar surface, characterized in that said second layer (13) of oxide has concentrations of phosphorus and boron of higher than 6% and lower than 3%, respectively; and said third layer (14) of oxide has concentrations of boron and phosphorus of 4-5% and 4%, respectively. The planarization process according to claim 1, characterized in that the thermal process is carried out at a temperature lower than 900 °C. The planarization process according to any one of the preceding claims, characterized in that the first (12), the second (13) and the third (14) oxide Layers are formed by means of a chemical vapor deposition technique. The planarization process according to any one of the preceding claims, characterized in that the first (12), the second (13) and the third (14) oxide layers have an overall thickness in the range 500 nm to 1500 nm. The planarization process according to claim 4, characterized in that the first layer (12) of oxide has a thickness in the range 50 to 300 nm. The planarization process according to claim 4, characterized in that the second layer (13) of oxide has a thickness in the range 80 nm to 500 nm. The planarization process according to claim 1, characterized in that after step d) the following steps are performed: e) selectively removing the first (12), second (13) and third (14) oxide Layers to open contact windows (15) wherein selected regions of the substrate (3) are exposed; f) depositing over the third oxide layer (14) a conductive material layer (17); g) selectively removing the conductive material layer (17) to form a desired pattern of electrical interconnection lines. An integrated circuit comprising a silicon substrate (3) wherein electronic devices (M,MC1,MC2) are obtained, comprising first-level electrical interconnection lines (4,11) over said substrate (3) and second-level electrical interconnection lines (17), the first-level (4,11) and second level (17) electrical interconnection lines being separated by an insulating layer (12,13,14), characterized in that the insulating layer (12,13,14) comprises a first undoped oxide layer (12) acting as a barrier layer superimposed over the first-level electrical interconnection lines (4,11), a second oxide layer (13) doped with phosphorus and boron superimposed over the first oxide layer (12) and acting as a gettering layer, and a highly planar third oxide layer (14) doped with boron and phosphorus, superimosed over the second oxide layer (13), characterized in that said second oxide layer (13) has concentrations of phosphorus and boron of higher than 6% and lower than 3%, respectively; and said third oxide layer (14) has concentrations of boron and phosphorus of 4-5% and 4%, respectively.

Inventors: BACCHETTA MAURIZIO, LOSAVIO ALDO, BACCHETTA, MAURIZIO, LOSAVIO, ALDO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1994
Main classification: H01L21
IPC codes: H01L29, H01L21, H01L23